A variety of medical diagnostic tests rely on identifying the presence and frequently the concentration of biological particles, such as different types of blood cells, bacteria and pathogens. Such tests include whole blood counting and concentration assessment of white blood cells for immunology-related diagnostics. Preferably, diagnostics systems quickly and inexpensively deliver accurate results without requiring highly trained technical staff. A flow cytometer is the standard diagnostics device for tests that rely on counting different types of blood cells. In a flow cytometer, a sample flows through a detection zone, typically one cell at a time, and cell properties are determined by an illumination and detection system. Based upon the measured properties, detected cells are grouped into categories to obtain the cell-type concentration of interest. Unfortunately, a flow cytometer is an expensive and complex instrument.